Preinstalled Android is not privacy friednly at all. It can leak location through any network, say through the network provided by iPhone. It can determine location from GPS, bluetooth and WiFi.

But for me the main downside of this approach is that you need to take two phones with you. I only have place for one in my pocket. It is not convenient to take two phones.

So my solution is to admit that any phone is infected by all kinds of googles and governments of the world and just take this into account. This means somebody else controls all the data on the phone, but they use there powers rarely